extend.texi

理解和实践操作系统的一本好书

                             &&hack - &&foo @};
goto *(&&foo + array[i]);
@end smallexample

@noindent
This is more friendly to code living in shared libraries, as it reduces
the number of dynamic relocations that are needed, and by consequence,
allows the data to be read-only.

The @code{&&foo} expressions for the same label might have different values
if the containing function is inlined or cloned.  If a program relies on
them being always the same, @code{__attribute__((__noinline__))} should
be used to prevent inlining.  If @code{&&foo} is used
in a static variable initializer, inlining is forbidden.

@node Nested Functions
@section Nested Functions
@cindex nested functions
@cindex downward funargs
@cindex thunks

A @dfn{nested function} is a function defined inside another function.
(Nested functions are not supported for GNU C++.)  The nested function's
name is local to the block where it is defined.  For example, here we
define a nested function named @code{square}, and call it twice:

@smallexample
@group
foo (double a, double b)
@{
  double square (double z) @{ return z * z; @}

  return square (a) + square (b);
@}
@end group
@end smallexample

The nested function can access all the variables of the containing
function that are visible at the point of its definition.  This is
called @dfn{lexical scoping}.  For example, here we show a nested
function which uses an inherited variable named @code{offset}:

@smallexample
@group
bar (int *array, int offset, int size)
@{
  int access (int *array, int index)
    @{ return array[index + offset]; @}
  int i;
  /* @r{@dots{}} */
  for (i = 0; i < size; i++)
    /* @r{@dots{}} */ access (array, i) /* @r{@dots{}} */
@}
@end group
@end smallexample

Nested function definitions are permitted within functions in the places
where variable definitions are allowed; that is, in any block, mixed
with the other declarations and statements in the block.

It is possible to call the nested function from outside the scope of its
name by storing its address or passing the address to another function:

@smallexample
hack (int *array, int size)
@{
  void store (int index, int value)
    @{ array[index] = value; @}

  intermediate (store, size);
@}
@end smallexample

Here, the function @code{intermediate} receives the address of
@code{store} as an argument.  If @code{intermediate} calls @code{store},
the arguments given to @code{store} are used to store into @code{array}.
But this technique works only so long as the containing function
(@code{hack}, in this example) does not exit.

If you try to call the nested function through its address after the
containing function has exited, all hell will break loose.  If you try
to call it after a containing scope level has exited, and if it refers
to some of the variables that are no longer in scope, you may be lucky,
but it's not wise to take the risk.  If, however, the nested function
does not refer to anything that has gone out of scope, you should be
safe.

GCC implements taking the address of a nested function using a technique
called @dfn{trampolines}.  A paper describing them is available as

@noindent
@uref{http://people.debian.org/~aaronl/Usenix88-lexic.pdf}.

A nested function can jump to a label inherited from a containing
function, provided the label was explicitly declared in the containing
function (@pxref{Local Labels}).  Such a jump returns instantly to the
containing function, exiting the nested function which did the
@code{goto} and any intermediate functions as well.  Here is an example:

@smallexample
@group
bar (int *array, int offset, int size)
@{
  __label__ failure;
  int access (int *array, int index)
    @{
      if (index > size)
        goto failure;
      return array[index + offset];
    @}
  int i;
  /* @r{@dots{}} */
  for (i = 0; i < size; i++)
    /* @r{@dots{}} */ access (array, i) /* @r{@dots{}} */
  /* @r{@dots{}} */
  return 0;

 /* @r{Control comes here from @code{access}
    if it detects an error.}  */
 failure:
  return -1;
@}
@end group
@end smallexample

A nested function always has no linkage.  Declaring one with
@code{extern} or @code{static} is erroneous.  If you need to declare the nested function
before its definition, use @code{auto} (which is otherwise meaningless
for function declarations).

@smallexample
bar (int *array, int offset, int size)
@{
  __label__ failure;
  auto int access (int *, int);
  /* @r{@dots{}} */
  int access (int *array, int index)
    @{
      if (index > size)
        goto failure;
      return array[index + offset];
    @}
  /* @r{@dots{}} */
@}
@end smallexample

@node Constructing Calls
@section Constructing Function Calls
@cindex constructing calls
@cindex forwarding calls

Using the built-in functions described below, you can record
the arguments a function received, and call another function
with the same arguments, without knowing the number or types
of the arguments.

You can also record the return value of that function call,
and later return that value, without knowing what data type
the function tried to return (as long as your caller expects
that data type).

However, these built-in functions may interact badly with some
sophisticated features or other extensions of the language.  It
is, therefore, not recommended to use them outside very simple
functions acting as mere forwarders for their arguments.

@deftypefn {Built-in Function} {void *} __builtin_apply_args ()
This built-in function returns a pointer to data
describing how to perform a call with the same arguments as were passed
to the current function.

The function saves the arg pointer register, structure value address,
and all registers that might be used to pass arguments to a function
into a block of memory allocated on the stack.  Then it returns the
address of that block.
@end deftypefn

@deftypefn {Built-in Function} {void *} __builtin_apply (void (*@var{function})(), void *@var{arguments}, size_t @var{size})
This built-in function invokes @var{function}
with a copy of the parameters described by @var{arguments}
and @var{size}.

The value of @var{arguments} should be the value returned by
@code{__builtin_apply_args}.  The argument @var{size} specifies the size
of the stack argument data, in bytes.

This function returns a pointer to data describing
how to return whatever value was returned by @var{function}.  The data
is saved in a block of memory allocated on the stack.

It is not always simple to compute the proper value for @var{size}.  The
value is used by @code{__builtin_apply} to compute the amount of data
that should be pushed on the stack and copied from the incoming argument
area.
@end deftypefn

@deftypefn {Built-in Function} {void} __builtin_return (void *@var{result})
This built-in function returns the value described by @var{result} from
the containing function.  You should specify, for @var{result}, a value
returned by @code{__builtin_apply}.
@end deftypefn

@deftypefn {Built-in Function} __builtin_va_arg_pack ()
This built-in function represents all anonymous arguments of an inline
function.  It can be used only in inline functions which will be always
inlined, never compiled as a separate function, such as those using
@code{__attribute__ ((__always_inline__))} or
@code{__attribute__ ((__gnu_inline__))} extern inline functions.
It must be only passed as last argument to some other function
with variable arguments.  This is useful for writing small wrapper
inlines for variable argument functions, when using preprocessor
macros is undesirable.  For example:
@smallexample
extern int myprintf (FILE *f, const char *format, ...);
extern inline __attribute__ ((__gnu_inline__)) int
myprintf (FILE *f, const char *format, ...)
@{
  int r = fprintf (f, "myprintf: ");
  if (r < 0)
    return r;
  int s = fprintf (f, format, __builtin_va_arg_pack ());
  if (s < 0)
    return s;
  return r + s;
@}
@end smallexample
@end deftypefn

@deftypefn {Built-in Function} __builtin_va_arg_pack_len ()
This built-in function returns the number of anonymous arguments of
an inline function.  It can be used only in inline functions which
will be always inlined, never compiled as a separate function, such
as those using @code{__attribute__ ((__always_inline__))} or
@code{__attribute__ ((__gnu_inline__))} extern inline functions.
For example following will do link or runtime checking of open
arguments for optimized code:
@smallexample
#ifdef __OPTIMIZE__
extern inline __attribute__((__gnu_inline__)) int
myopen (const char *path, int oflag, ...)
@{
  if (__builtin_va_arg_pack_len () > 1)
    warn_open_too_many_arguments ();

  if (__builtin_constant_p (oflag))
    @{
      if ((oflag & O_CREAT) != 0 && __builtin_va_arg_pack_len () < 1)
        @{
          warn_open_missing_mode ();
          return __open_2 (path, oflag);
        @}
      return open (path, oflag, __builtin_va_arg_pack ());
    @}
    
  if (__builtin_va_arg_pack_len () < 1)
    return __open_2 (path, oflag);

  return open (path, oflag, __builtin_va_arg_pack ());
@}
#endif
@end smallexample
@end deftypefn

@node Typeof
@section Referring to a Type with @code{typeof}
@findex typeof
@findex sizeof
@cindex macros, types of arguments

Another way to refer to the type of an expression is with @code{typeof}.
The syntax of using of this keyword looks like @code{sizeof}, but the
construct acts semantically like a type name defined with @code{typedef}.

There are two ways of writing the argument to @code{typeof}: with an
expression or with a type.  Here is an example with an expression:

@smallexample
typeof (x[0](1))
@end smallexample

@noindent
This assumes that @code{x} is an array of pointers to functions;
the type described is that of the values of the functions.

Here is an example with a typename as the argument:

@smallexample
typeof (int *)
@end smallexample

@noindent
Here the type described is that of pointers to @code{int}.

If you are writing a header file that must work when included in ISO C
programs, write @code{__typeof__} instead of @code{typeof}.
@xref{Alternate Keywords}.

A @code{typeof}-construct can be used anywhere a typedef name could be
used.  For example, you can use it in a declaration, in a cast, or inside
of @code{sizeof} or @code{typeof}.

@code{typeof} is often useful in conjunction with the
statements-within-expressions feature.  Here is how the two together can
be used to define a safe ``maximum'' macro that operates on any
arithmetic type and evaluates each of its arguments exactly once:

@smallexample
#define max(a,b) \
  (@{ typeof (a) _a = (a); \
      typeof (b) _b = (b); \
    _a > _b ? _a : _b; @})
@end smallexample

@cindex underscores in variables in macros
@cindex @samp{_} in variables in macros
@cindex local variables in macros
@cindex variables, local, in macros
@cindex macros, local variables in

The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within the
expressions that are substituted for @code{a} and @code{b}.  Eventually we
hope to design a new form of declaration syntax that allows you to declare
variables whose scopes start only after their initializers; this will be a
more reliable way to prevent such conflicts.

@noindent
Some more examples of the use of @code{typeof}:

@itemize @bullet
@item
This declares @code{y} with the type of what @code{x} points to.

@smallexample
typeof (*x) y;
@end smallexample

@item
This declares @code{y} as an array of such values.

@smallexample
typeof (*x) y[4];
@end smallexample

@item
This declares @code{y} as an array of pointers to characters:

@smallexample
typeof (typeof (char *)[4]) y;
@end smallexample

@noindent
It is equivalent to the following traditional C declaration:

@smallexample
char *y[4];
@end smallexample

To see the meaning of the declaration using @code{typeof}, and why it
might be a useful way to write, rewrite it with these macros:

@smallexample
#define pointer(T)  typeof(T *)